The present invention relates to a hydraulic control system for an automatic transmission, and more particularly to a less complicated valving circuit for determining a desired shift schedule in an automatic transmission.
A known hydraulic control system for automatic transmission is disclosed on Page 56 of a publication entitled "THM700-R4 PRINCIPLES OF OPERATION" published by General Motors in July 1983. In this known hydraulic control system, a modulated throttle valve up and a modulated throttle valve down are provided in addition to a throttle valve. The modulated throttle valve down reduces a throttle pressure generated by the throttle valve to generate a modulated throttle valve down pressure which is a relatively high throttle related pressure. The modulated throttle valve up reduces the throttle pressure to generate a modulated throttle valve up pressure which is a relatively low throttle related pressure. The modulated throttle valve down pressure acts on a 3-4 shift valve against governor pressure when the shift valve is in a down position thereof, whereas the modulated throttle valve up pressure acts on the 3-4 shift valve against governor pressure when the shift valve is in an up position thereof, inducing hysteresis between upshift and downshift of the shift valve. In this hydraulic control system, the shift valve is subject also to a partial throttle pressure. The partial throttle pressure is zero at throttle opening degrees ranging from zero to half (1/2) of the wide open throttle opening degree, and becomes as high as the throttle pressure at throttle opening degrees ranging from the half to the wide open throttle opening degree. Considering now an upshift schedule line and a downshift schedule line drawn in a graph having throttle opening values on a vertical axis and vehicle speed values on a horizontal axis, these lines are discontinuous in the vicinity of the half (1/2) of the wide open throttle opening degree. Within a range where the throttle opening degree is smaller than the half of the wide open throttle opening degree, the shift schedule lines are shifted toward low vehicle speed side, whereas within a range where the throttle opening degree is larger than the half of the wide open throttle opening degree, they are shifted toward high vehicle speed side. These shift schedule lines are shown in FIG. 6. As shown in FIG. 6, that portion of the downshift schedule line (drawn by broken line) which results from connecting downshift points at throttle opening degrees ranging from the half (1/2) to the wide open throttle opening degree is disposed on the higher vehicle speed side than that portion of the upshift schedule line (drawn by full line) which results from connecting upshift points at throttle opening degrees ranging from zero throttle opening degree to the half (1/2) of the wide open throttle opening degree. Thus, the hysteresis disappears at certain vehicle speeds during operation with a throttle opening degrees around the half (1/2) of the wide open throttle opening degree. This is indicated by the reference character G in FIG. 6. This is where the shift valve tends to undergo hunting.
As shown in FIG. 6, the downshift and upshift schedule lines are similar in shape, so that hysteresis at small throttle opening degrees and that at large throttle opening degrees are basically alike. Thus, appropriate setting of hysteresis over the whole range of throttle openings is very difficult. It is preferrable that the downshift schedule line be shifted toward low vehicle speed side as being sufficiently separated from the upshift schedule line at small throttle opening degrees to provide an appropriate hysteresis. However, if it is so set, the downshift schedule line is separated too much from the upshift schedule line at large throttle opening degrees. This provides inappropriate substantially great hysteresis.
In the known hydraulic control system, two modulated throttle valves are required to generate the modulated throttle valve up and down pressures which are lower than the throttle pressure. This results in complicated circuit structure.
An object of the present invention is to provide a hydraulic system for controlling a shift in an automatic transmission such that, with reduced number of valves and thus with simple circuit structure, the above-mentioned problems are solved.